The present invention relates to a fixture and method for measuring the end face geometry and angle of polish of guide pin based multi-fiber fiber optic connectors using an interferometric microscope.
Optical fibers allow the transmission of data over great distances using light signals. The basic construction of the optical fiber which is made of either glass, a polymeric material or a combination of both, includes a small core, surrounded by a lower index cladding material. The light signal travels through the core and if it travels to the edge of the core, the refractive index of the cladding causes it to reflect back toward the center and continue down the fiber core. A high performance fiber optic connection is made between two fibers when the fiber ends are brought into contact with the cores in precise axial alignment so that the light signal can pass from one fiber core end to the other.
Multifiber ferrules (illustrated in FIG. 1) or connectors allow anywhere from 2 to 72 fibers in a single ferrule to be intermated with one another. These high densities offer advantages in time, money and size for the growing demands of data hungry applications. Guide pin based multifiber connectors are intermated with the use of guide holes which are molded or machined symmetrically on both edges of the connectors and use precision machined, matching guide pins that slide tightly into the guide holes and provide a common axis for the guide holes of mating connectors.
In this patent, the terms “ferrule” and “connector” are used interchangeably, with the “ferrule” typically referring to the central element of the connector which includes the fibers and the guide holes and the assembled “connector” which refers to the polished ferrule with some type of connector housing to provide stability for when they are mated.
In general, manufacturers desire to make multifiber connectors with the greatest efficiency in light transmission. The transmission loss of the connector at the fiber to fiber interface is attributed to three main factors: 1. Transverse offset of the fibers; 2. Fiber end gap; and 3. Mechanical instability.
The transverse offset is the error due to lateral misalignment of the fiber optic cores. This is controlled by dimensional tolerances in the manufacturing of the fiber and the ferrule and the guide pins. Fiber end gap is the error due to the fiber tips not forming the intimate optical contact with each other as required for good transmission. If the fibers do not make good contact, an air gap is formed that causes light to be reflected back into the system (back reflection) which can deteriorate the quality of the signal. The mechanical stability is related to the end face angle of the connectors. If the connectors are not polished reasonably flat on the end faces with the fibers, the ability of the connectors to form a stable connection when intermated is affected.
Polishing methods for the connector end faces are still being optimized. Depending on the performance requirements, the final connector end face geometry may be a flat polish, a protruded fiber polish or an angled protruded fiber polish. Ideally, for good physical contact, the fiber ends of the connector should be in the same plane. For mechanical stability, the critical requirement is that the end face geometry must be controlled to allow the fibers, and not the ferrule, to be the first to contact. When two connectors mate in this manner, the fibers align with each other and compress uniformly to provide controlled, intimate, optical contact.
There has long been a need to inspect the end of a guide pin based multifiber optical connector for information on the geometry of the end face to control the polishing process and to confirm quality for assurance of performance in an application. Typical parameters to be measured are: radius of curvature of the ferrule and fiber end, angle of the end face of the ferrule, symmetry of the polish, and any undercut or protrusion of the fibers with reference to the ferrule surface
Interferometry is one of the preferred methods for measuring the surface of a multiple fiber optical connector because the resulting fringe pattern provides three dimensional information about the surface of the connector, and allows the above-described parameters to be accurately estimated or calculated. In order to use interferometry to measure the end face geometry and angle of the polish of a multiple fiber optical connector, it is necessary to stably hold the connector in a known position which is both precise and repeatable and typically is perpendicular, with respect to a reference surface in an interferometer. Such precision positioning is necessary to provide consistent, accurate, and reproducible measurements whenever a connector is inserted into a measuring instrument.
For example, MT or MiniMT type guide pin based connectors having a flat or slightly domed shaped end face are intended to be polished so that the optical fibers are slightly protruding from the ferrule surface. When two MT connectors are brought into contact, two guide pins which have been preinserted in predetermined locations in one connector mate with two guide holes which have been established at predetermined corresponding locations in the opposing connector thereby providing precise alignment of the fiber cores and holding both connectors perpendicular to each other. At the same time, the fibers in both connectors contact one another and compress slightly to form intimate optical contact which minimizes reflections at the interface. In order to guarantee that the fibers make contact first without any interference from the ferrule, a typical standard for end face geometry allows from 1 to 3 microns of fiber protrusion and an end face angle in the X and Y direction of 0.20° or less.
Industry standards recognize that the ideal end face surface of the connector should be a plane precisely perpendicular to the guide holes since these determine the angle on contact. So all angular measurements of the end face surface must use the guide holes as a reference. Since the guide holes themselves can not be readily measured, guide pins must be used in some manner to reference the guide holes.
Because of the tight tolerances required in the manufacturing process, and the need to hold such tolerances to a hundredth of a degree of accuracy, the slightest variation in ferrule position from a precise and repeatable axis with respect to the reference surface will lead to varying measurements depending on the position of the ferrule around the actual axis. In the prior art, this problem has been addressed with some success by using reference guide pins which has been machined or polished “flat”. In other words, the end of the guide pin is machined or polished to form a flat surface which is designed to be perpendicular to the cylindrical sides of the pin. The reference pins are inserted into the connector guide holes before placing the connector onto the interferometer. When the interferometer provides the three dimensional information about the end face of the connector, the average angle of the two “flat” guide pins are used as a reference plane and is compared to the best fit plane of the ferrule end face to calculate the angle of the end face surface.
The difficulty with this prior art method is in the inability to get a repeatable reference surface or “flat” guide pin. The typical guide pin is only 0.7 mm in diameter. It can vary in length from 3 to 12 mm. Even with extreme precision machining, it is not always possible to get the end of such a tiny cylindrical pin to be perfectly flat. The problem is there is no reference surface on the cylindrical walls with which to guarantee the position either in manufacturing or when trying to measure. Since it is not possible to measure the accuracy of the flatness of the specially prepared guide pins, the prior art method is only a relative method. There is no true reference or method available to certify the measurement. Each one is different and the measurements will vary depending on how the guide pins are inserted into the guide holes. The direction of the angle of the “flat” pins in the X and Y axis is used to compare it with the X and Y angle of the measured connector end face. Since two guide pins are used, each one contributes its own variations and the error could be doubled.
Another difficulty with the prior art method is the general handling of the tiny components. It is not easy to manually hold the guide pins for insertion or for removing from the guide holes in the connectors. Fixturing has been designed to hold the pins for insertion. This helps but it requires extra steps and more time for each connector before the measurement process can begin.